Ajax incompatibility?

I have an Add to Cart form for Drupal Commerce which includes a Signature Field.

The user can pick which product to add to the cart by using a select field. When the user changes the selected product, an ajax call regenerates the add to cart form.

Problem:
When the user first visits the page the signature field works fine. But if they change the selected product and the form is regenerated by ajax, the signature field stops working. (Grey signature line disappears, text box does not create the faux signature, and the field fails to validate.)

For forms that regenerate with ajax, does another function need to be added to the form to re-initialize the signature field?